Method for making electro-optical device

ABSTRACT

An electro-optic device, comprising a layer of light-carrying material; and a rib, projecting from the layer of light-carrying material, for guiding optical signals propagating through the device. The layer of light-carrying material comprises a first doped region of a first type extending into the rib, and a second doped region of a second, different type extending into the rib such that a pn junction is formed within the rib. The pn junction extends substantially parallel to at least two contiguous faces of the rib, resulting in a more efficient device. In addition, a self-aligned fabrication process can be used in order to simplify the fabrication process and increase reliability and yield.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to Great Britain applicationGB1002726.6 filed Feb. 17, 2010 and to PCT application PCT/GB2011/000216filed Feb. 17, 2011. This patent application is a divisional applicationof U.S. Ser. No. 13/578,837 filed Aug. 14, 2012.

FIELD OF THE INVENTION

The present invention relates to the field of electro-optics, andparticularly to electro-optic devices comprising a waveguide rib andmethods for the fabrication thereof.

BACKGROUND ART

Silicon microphotonics has generated an increasing interest in recentyears. Integrating optics and electronics on the same chip would allowenhancement of integrated circuit (IC) performance. Furthermore,telecommunications could benefit from the development of low costsolutions for high-speed optical links. The realization of activephotonic devices, in particular high speed optical modulators integratedin silicon-on-insulator (SOI) waveguides, is essential for thedevelopment of silicon microphotonics/nanophotonics.

Although silicon does not in normal circumstances exhibit a linearelectro-optic (Pockels) effect, other mechanisms are available formodulation, including thermo-optic and plasma dispersion effects. Asidefrom these, further interesting methods have been reported which includeusing strain to introduce a Pockels effect, forming SiGe/Ge quantumwells to take advantage of the quantum-confined stark effect, andbonding III-V materials to make use of their stronger electro-opticproperties. The disadvantage of these approaches is the complex ornon-CMOS compatible fabrication processes involved. The thermo-opticeffect in silicon is relatively, very slow and therefore has no real usefor high speed applications. The plasma dispersion effect on the otherhand is much more promising with most of the recent successfulhigh-speed silicon modulators being based upon this effect, whilst usingcarrier injection, depletion or accumulation to cause the requiredchanges in free-carrier concentration.

The plasma dispersion effect uses changes in the free-carrierconcentration to cause modulation of the light passing through thedevice. The free-carrier concentration may be changed by injectingcarriers into the device, depleting carriers from a region of the deviceor by causing an accumulation of charge carriers in a region of thedevice. Carrier injection is typically carried out in a PIN diodestructure with the optical waveguide passing though the intrinsicregion. When the diode is forward biased, carriers pass into theintrinsic region causing a change in refractive index. Carrier depletioncan be based upon a PN junction diode in the waveguide. Reverse biasingthe diode causes carriers to be swept out of part or all of thewaveguide region, again resulting in a change in refractive index.Carrier accumulation involves the use of an insulating layer between Pand N diode regions that will, when biased, cause an accumulation offree carriers on the edges of the layer, much like a capacitor. Carrierdepletion and accumulation, unlike carrier injection, are not limited bythe relatively long minority carrier lifetime in silicon andconsequently the fastest reported devices have utilised thesemechanisms.

The figures of merit for classifying optical modulators are as follows:

-   -   Electro-optic bandwidth: this indicates the high-speed cut off        frequency and can be used to predict data transmission rates in        the absence of an eye diagram.    -   Data transmission rate: this indicates the rate at which data        can be transmitted, with 5 Gb/s, 10 Gb/s or 40 Gb/s normally        being targeted.    -   Dynamic extinction ratio: this gives the difference between the        modulators on and off power levels at a specified data rate. A        large extinction ratio will allow for longer transmission        lengths before data restoration is required.    -   Optical insertion loss.    -   DC extinction ratio: this indicates the low speed difference in        on and off power levels.    -   VπLπ efficiency: since devices produce phase modulation which is        later converted to intensity modulation, this describes the        voltage-length product required to produce a π radian phase        shift.    -   Size.    -   Power efficiency.

Other than these quantifiable factors, however, it is also important toconsider the ease of fabrication and expected tolerances in deviceperformance caused by slight variations inherent in the fabricationprocesses used, as these can have a direct effect on production cost anddevice yield. Existing devices have improved the data transmission rateand VπLπ efficiency, but are not always practical for mass productiondue to their complex structure.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide anelectro-optic device, such as an electro-optical modulator, that isrelatively simpler and easier to produce than existing devices, butnonetheless provides strong performance in terms of the quantifiablefactors listed above.

In one aspect, the present invention provides an electro-optic device,comprising a layer of light-carrying material; and a rib, projectingfrom the layer of light-carrying material, for guiding optical signalspropagating through the device. The layer of light-carrying materialcomprises a first doped region of a first type extending into the rib,and a second doped region of a second, different type extending into therib such that a pn junction is formed within the rib, substantiallyparallel to at least two contiguous faces of the rib.

In one embodiment, the pn junction runs substantially parallel to threefaces of the rib.

These structures have the advantage that they allow a self-alignedfabrication process, increasing yield, and the greater surface area ofthe pn junction increases the efficiency of the device.

Thus, according to another aspect of the present invention, there isprovided a method of fabricating an electro-optic device, comprising:depositing a mask over an area of light-carrying material and partiallyetching parts of the light-carrying material not covered by the mask, toform thereby a layer of light-carrying material and a rib, projectingfrom the layer of light-carrying material, for guiding optical signalspropagating through the device, said area of light-carrying materialcomprising a first doped region of a first type and a second dopedregion of a second, different type, at least partially overlaying thefirst doped region; and doping a side portion of the rib, abutting saidmask, with dopants of said second type, such that said second dopedregion is extended and a pn junction is formed within the ribsubstantially parallel to at least two contiguous faces of the rib.

As mentioned above, the mask may be used as a guide for both the etchingprocess to define the waveguide rib, and the doping process to form thepn junction within the waveguide rib. Thus the process is self-aligned,resulting in increased yield.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way ofexample, with reference to the accompanying figures in which:

FIG. 1 shows an electro-optic device according to embodiments of thepresent invention;

FIGS. 2a to 2h show a method of fabricating the electro-optic device asshown in FIG. 1;

FIG. 3 is a flowchart of a method in accordance with embodiments of thepresent invention; and

FIG. 4 is a graph of preliminary results for the electro-optic bandwidthof the device shown in FIG. 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

As stated above, conventional silicon electro-optic devices (such aselectro-optic modulators) comprise a waveguide portion through whichphotons are transmitted. In operation, the free carrier concentration ofthe waveguide portion may be adjusted in order to change its refractiveindex and control the passage of light through the device. This isusually achieved via a pn or pin junction, or MOS capacitor formedacross/in the waveguide. However, the design of conventional devices hasbeen such that their fabrication is difficult to achieve in CMOSprocessing.

According to embodiments of the present invention, an electro-opticdevice comprises a layer of light-carrying material, such as silicon,and a rib projecting therefrom. The layer has two doped regions ofopposite type, with both doped regions extending into the rib andarranged such that a pn junction is formed which is substantiallyparallel to at least two faces of the rib. This structure has theadvantage that it allows a self-aligned fabrication process, increasingreliability and yield, and the greater area of the pn junction increasesthe efficiency of the device.

FIG. 1 shows in cross-section an electro-optic modulator 110 accordingto an embodiment of the present invention (not to scale).

The modulator 110 comprises a substrate 112 and an insulating layer 114formed thereon. In one embodiment, the substrate is fabricated fromsilicon and the insulating layer from silicon dioxide; the insulatinglayer 114 is frequently referred to in the art as a ‘buried oxide’layer. However, alternative materials will be apparent to those skilledin the art, such as (but not limited to) silicon on sapphire (where theentire sapphire substrate is insulating), germanium on silicon, andsilicon-germanium on insulator.

Above the insulating layer 114 is formed a layer of light-carryingmaterial. In one embodiment, the light-carrying material is silicon andthe layer is primarily composed of nominally intrinsic silicon 116.However, formed within the layer are two regions of doped material: ap-type doped region 118 and an n-type doped region 120. The two dopedregions 118, 120 abut each other to form a pn junction, as is well knownin the art, and the n-type region 118 overlaps the p-type region 120 sothat the pn junction is formed both above and to the side of the p-typeregion. Each region 118, 120 is further divided into two parts. Thep-type region 118 comprises a first part 118 a of highly doped material(as commonly denoted p+ in the literature), and a second part 118 b ofrelatively sparsely doped material (as commonly denoted p in theliterature), that is, sparsely doped relative to the highly doped region118 a. Similarly, the n-type region 120 comprises a first part 120 a ofhighly doped material (as commonly denoted n+ in the literature), and asecond part 120 b of relatively sparsely doped material (as commonlydenoted n in the literature), that is, sparsely doped relative to thehighly doped region 120 a. The pn junction between the two regions 118,120 is formed between the relatively sparsely doped regions 118 b, 120b. The p- and n-type regions 118 b, 120 b are typically doped at aconcentration of between about 10¹⁷ and 10¹⁹ cm⁻³; and the p+ and n+regions 118 a, 120 a doped at a concentration of between about 10¹⁹ and10²⁰ cm⁻³, although different concentrations may be used, and the rangesmay overlap. It will be appreciated that the terms n and n+(andsimilarly p and p+) are used to denote differences in the carrierconcentration rather than absolute concentrations. The absoluteconcentrations may be tailored as desired in order to achieve a certainperformance characteristic. An example of a possible p-type dopant isboron, and possible n-type dopants include phosphorus, antimony andarsenic. It should also be noted that the nominally intrinsic regions116, will typically contain residual dopants and may therefore belightly doped p-type or lightly doped n-type (typically 10¹⁵ cm⁻³).

The device 110 further comprises a rib 121 (indicated by the dashed linein FIG. 1) for guiding optical signals propagating through the device(in many optical modes photons will also propagate in areas outside thewaveguide rib 121). The p-type region 118 b comprises a relativelythicker portion that extends into the rib. The n-type region 120 b alsoextends into the rib 121, abutting the p-type region 118 b such that apn junction is formed between them. Nonetheless, a junction may be saidto form between the two regions 118 b, 120 b. According to embodimentsof the present invention, the pn junction so formed is arranged to besubstantially parallel to at least two contiguous faces of the rib 121.In the illustrated embodiment, the pn junction is substantially parallelto three faces of the rib 121, i.e. the n-type region 120 b surroundsthe thicker portion of the region 118 b such that the pn junctionbetween the two runs substantially parallel to one of the “side” faces,then substantially parallel to the “top” face, and finally parallel tothe other “side” face of the rib 121. As will be explained in detailbelow, this structure lends itself to a particularly simple and reliablefabrication method. In addition, the device 110 is made more efficientby extending the pn junction around more than one face of the rib 121.

A further insulating layer 122 is formed over the light-carrying layerand waveguide rib 121. The further insulating layer 122 may again befabricated from silicon dioxide. Contacts 124, 126 are formedrespectively with the p+ and n+ regions 118 a, 120 a, by passingconducting material (e.g. metallic materials) through the insulatinglayer 122. One of the contacts (e.g. contact 124) is connected to areference voltage such as ground, and the other (e.g. contact 126) isconnected to a signal, such that an electric potential difference can beapplied to bias the pn junction as desired. It will also be apparentthat, if operated in push pull with a modulator in each arm of aMach-Zehnder interferometer for example, electrical potentials may beapplied to both, or neither of the contacts 124, 126. The presentinvention is not limited to any particular biasing scheme.

The insulating layer 122 is not essential for the device to work, but isused as an upper cladding to both protect the waveguide rib 121 and topassivate the surface so that surface traps which would collect carriersare minimized. In the case of a modulator, it isolates the electricalcontact regions 124, 126, and allows connections to the contact to belaid across the top of the insulating layer 122 without affecting otherparts of the waveguide. The device would also work with an air cladding,however (i.e. without the insulating layer 122).

FIGS. 2a to 2h show the steps in a method of fabrication of themodulator 110 as shown in FIG. 1. FIG. 3 shows the method in the form ofa flowchart.

FIG. 2a shows a substrate (e.g. a silicon substrate) 112 covered by alayer 114 of insulating material (e.g. silicon dioxide), with a layer oflight-carrying material (e.g. silicon) on the insulator layer 114. Sucha combination is commonly referred to as silicon on insulator (SOI), andis readily available. The light-carrying layer is covered by a furtherlayer of insulating material 122 (e.g. silicon dioxide), and a layer ofresist 128 is deposited and patterned onto this insulating layer (seestep S10, part). The insulating layer 122 is deposited before the dopingof the p-type and n-type regions 118, 120 described below, to avoid theroughening of the silicon layer forming the top of the waveguide (insubsequent method steps) that would otherwise occur, leading toincreased optical losses. The resist 128 defines a window which is inturn used to define doped regions within the layer of light-carryingmaterial. In the illustrated embodiment, n-type dopants 142 (e.g.phosphorus, antimony or arsenic) are used in the background (at arelatively low energy), and p-type dopants 140 (e.g. boron) areimplanted (at a relatively high energy) to generate respectively anupper n-type region 120 and a lower p-type region 118 (see step S10,part). The dopants are deposited into the uncovered region at aconcentration of between about 10¹⁷ and about 10¹⁹ cm⁻³, and theremaining resist 128 subsequently removed. The regions 116 of thelight-carrying layer covered by the resist 128 are left undoped, i.e.nominally intrinsic (in practice a small number of dopants may bepresent in these regions through diffusion, for example). In analternative embodiment, the doped regions 118, 120 may be formeddirectly in the light-carrying layer, and the insulating layer 122subsequently deposited over the light-carrying layer.

However, such doping may be achieved in any one of a number of ways thatwill be familiar to those skilled in the art. Alternative methodsinclude (but are not limited to): epitaxially growing the layer(providing there is a seed layer) with doping ready incorporated(in-situ doping); depositing doped amorphous layers and performing solidphase epitaxial regrowth (providing there is a seed layer);plasma-immersion; and in-diffusion of doping.

FIG. 2b shows the next stage in which the insulating layer 122 has beenthickened, so that subsequent etching processes do not completely removethe layer 122 (see FIG. 2c below). In addition, by thickening theinsulating layer 122 at this stage, after doping of the p-type andn-type regions 118, 120, a lower implantation energy may be used in theprevious step than would otherwise be the case. It will be apparent tothose skilled in the art, however, that the insulating layer 122 mayalso be deposited in its full thickness in a single step, before orafter the doping of the p-type and n-type regions 118, 120.

Two areas of the insulating layer have been etched to define an areawhere the waveguide rib part of the device is to be formed (see stepS12). This structure may be achieved, for example, by depositing andpatterning a further layer of resist to leave uncovered two channelseither side of the site of the waveguide rib. An etch process may thenbe used to etch away the insulating layer in those regions.

FIG. 2c shows the next stage in the process, in which a further etchingprocess has been used to partially etch away two areas of thelight-carrying layer so as to leave a waveguide rib therebetween,leaving relatively thin layers of intrinsic material 116 and p-typematerial 118 at the bottom of each channel. This process also etches theinsulating layer 122, and so the insulating layer 122 is thinner afterthis step. In between the channels, a relatively thick layer of p-typematerial is left, and this helps to define the waveguide rib 121(indicated by dashed lines in FIG. 2c ) in combination with a layer ofn-type material 120 above.

FIG. 2d shows the next stage in the process, where one of the channelshas been covered by depositing and patterning a layer of resist 130(step S14). The other channel is doped with two dopants (step S16).p-type dopants 144 are implanted at an angle substantially perpendicularto the plane of the device, and therefore ensure that the bottom of thatchannel is of p-type material. That is, the p-type region 118 extendsinto that channel. n-type dopants 146 are implanted at an angle relativeto the p-type dopants 144 towards the waveguide rib 121. It should benoted that the p-type dopants 144 are implanted at a concentration thatis high enough not to be compensated for by the n-type dopants 146. Inaddition, the n-type dopants 146 are implanted with a sufficiently lowenergy that the p-type region 118 extends continuously from the bottomof the channel into the rib 121, i.e. the p-type region at the bottom ofthe channel is not separated from the p-type region in the rib 121 by anarea of n-type doping.

In this way, the side of the waveguide rib 121 is implanted with n-typedopants such that the n-type region 120 extends around the p-type region118 (see FIG. 2e for the resulting doped regions). The insulating layer122 is used as part of the mask defining the implantation regions,meaning that the resist 130 may be patterned with a more relaxedtolerance which depends on the width of the waveguide rib (i.e. thedopants 144 and 146 are self aligned by the insulating layer 122).

In an alternative embodiment, the n-type dopants 146 may be omitted fromthe method. As will be clear from the description below, this embodimentresults in a pn junction which runs substantially parallel to just twocontiguous faces of the rib, i.e. the “top” face of the rib 121 and justone of the “side” faces.

FIG. 2e shows the next stage in the process, in which similar steps areperformed on the second channel. The first channel is covered bydepositing and patterning a layer of resist 132 (step S18), and thesecond channel is implanted with n-type dopants at two angles (stepS20): first dopants 148 at an angle substantially perpendicular to theplane of the device, and second dopants 150 at an angle relative to thefirst dopants 148 towards the waveguide rib 121. The first dopants 148ensure that the bottom of the second channel is n-type, and the seconddopants 150 ensure that the side of the waveguide rib 121 is alson-type, i.e. so that the n-type region 120 extends around the waveguiderib 121 on three faces and into the bottom of the second channel (seeFIG. 2f for the finished doped regions). This time, in contrast to thepreceding step, dopants 148, 150 are implanted with sufficient energyand concentration that the area between the side of the rib 121 and thebottom of the channel is continuously n-type, i.e. the p-type region isconverted to (net) n-type so that the n-type region 120 extendscontinuously from the bottom of the channel to the side of the rib 121.Dopants 148, 150 can also be replaced by steps of plasma immersion. Thisprocess creates a continuous doped region on the side of the rib 121 andthe bottom of the second channel in a single step.

Again, it should be noted that the insulating layer 122 is used as partof the mask defining the implantation regions, meaning that the resist132 may be patterned with a more relaxed tolerance which depends on thewidth of the waveguide rib (i.e. the dopants 148 and 150 are selfaligned by the insulating layer 122).

The above steps therefore show a method of fabricating an electro-opticdevice 110 in which the pn junction is formed substantially parallel to(up to) three sides of the waveguide rib, increasing efficiency of thedevice. In addition, the device structure allows the use of aself-aligned fabrication process, increasing device reliability andyield.

FIGS. 2f and 2g show stages in which n+ and p+ regions are created inthe channels, for forming electrical contacts with metallic materials inorder to bias the pn junction. In FIG. 2f , a layer of resist 134 isdeposited and patterned to define a relatively narrow window within thefirst channel (step S22). p-type dopants 152 are implanted at arelatively high concentration into this window (e.g. between about 10¹⁹and 10²⁰ cm⁻³), to define a p+ region 118 a (step S24). The relativelysparsely doped p-type region is denoted 118 b. In FIG. 2g , a layer ofresist 136 is deposited and patterned to define a relatively narrowwindow within the second channel (step S26). n-type dopants 154 areimplanted at a relatively high concentration into this window (e.g.between about 10¹⁹ and 10²⁰ cm⁻³), to define an n+ region 120 a (stepS28). The relatively sparsely doped n-type region is denoted 120 b. Atany stage after the doped regions have been created, a thermal processmay be used to electrically activate the dopants (e.g. a rapid thermalanneal at 1000° C. or higher, for a short period of 15 seconds to limitdiffusion of the dopants).

FIG. 2h shows the finished device, in which further insulating materialhas been deposited over the device to greatly increase the thickness ofthe insulating layer 122 (step S30). Windows are opened in the oxide ontop of the p+ and n+ regions 118 a and 120 a. A stack of differentmetals is deposited on top of the wafer and a metal etch is performed todefine the electrodes 124, 126 (step S32). The electrodes 124, 126 maybe formed using any conducting material, however, and may comprise asingle material rather than a stack of materials.

It will be apparent to those skilled in the art that various alterationscan be made to the device and method disclosed above without departingfrom the scope of the invention. For example, the relative positions ofthe n- and p-type regions may be reversed, i.e. the thicker doped regionwithin the waveguide rib may be formed from n-type material, and thesurrounding region from p-type material. Similarly, the steps as set outabove need not be performed in the strict order disclosed. For example,steps S14 and S16 may be performed after steps S18 and S20; similarly,steps S22 and S24 may be performed after steps S26 and S28.

In addition, it will also be apparent to those skilled in the art thatthe depiction of the device in the drawings is idealized for thepurposes of clarity. In particular, the drawings tend to show regionswith sharply defined boundaries between them, and right-angled corners,etc. In reality, however, the geometry of electro-optic devices on thescale of hundreds of manometers (as in embodiments of the presentinvention) is not so precisely defined. For example, the method ofdoping may be such that the boundaries of the doped regions are moreobtuse or transitional than illustrated (for example, implantationresults in a distribution of dopants rather than an abrupt interface).Dopants may also diffuse from their starting positions. Adjacent p typeand n-type regions may therefore overlap slightly (i.e. an area may haveboth n-type and p-type dopants), or conversely not be in direct contact.Therefore, the p-type region 118 b and the n-type region 120 b only“abut” to the extent that a pn junction (and the resulting depletionregion) is formed between them. In addition, the rib 121 may not have anexactly rectangular cross-section; instead the faces of the rib may becurved, or otherwise irregular, and the corners thereof may be roundedand may be other than right angles. The use of the hard mask 122 to bothdefine the rib 121 and the doping of regions inside the rib, however,does mean that there will be a change in angle between the sides of therib and the top of the rib. Thus, the rib can be said to have faces,regardless of whether those faces are curved or flat, and the pnjunction found within the rib extends substantially parallel to at leasttwo of those faces.

Simulation results predicted an electro-optic bandwidth above 50 GHz,and preliminary results have demonstrated an electro-optic bandwidth ofapproximately 32 GHz for a 910 microns long device. This is shown inFIG. 4. Thus it can be seen that good performance is achieved in thedevice.

There is thus described an electro-optic device comprising a layer oflight-carrying material and a rib projecting therefrom. A pn junctionformed between oppositely doped regions extending from the layer intothe rib runs parallel to at least two faces of the waveguide rib. Inthis way, the efficiency of the pn junction and the device as a whole isimproved. The device structure also lends itself to a self-alignedfabrication process that increases device reliability and yield.

It will of course be understood that many variations may be made to theabove-described embodiment without departing from the scope of thepresent invention. For example, those skilled in the art will appreciatethat many of the method steps set out in the application may beperformed in an alternative order without departing from the scope ofthe present invention.

What is claimed is:
 1. A method of fabricating an electro-optic device,comprising: on a substrate forming a first structure comprising a firstinsulating layer, a first layer, a second layer, and a light carryinglayer, wherein the light carrying layer is deposited or grown atop thefirst insulating layer, and the first layer and the second layer aresubsequently deposited or grown atop the first insulating layer;depositing or growing a second insulating layer atop the firststructure, and depositing one or more first masks atop the secondinsulating layer; doping the first and the second layers through awindow created by the one or more first masks; wherein the first layeris doped with a first dopant and the second layer is doped with a seconddopant; wherein the first dopant and the second dopant are of anopposite type, thus forming a p-n junction between the first layer andthe second layer; etching the first mask, and then depositing or growinga third insulating layer thus thickening the second insulating layer;etching two portions of the second insulating layer entirely through thesecond insulating layer and further partially through the first layer,thus creating a first channel and a second channel; depositing a secondmask over the second channel; doping the first channel, abutting aregion formed by the second insulating layer and the second mask, withthe first and the second dopants; wherein the first dopants are directedperpendicular relative to a top plane of the device and wherein thesecond dopants are directed at a non-zero angle relative to the firstdopants; depositing a third mask in the first channel, and then dopingthe second channel, abutting a mask formed by the second insulatinglayer and the third mask, with the second dopants in two cycles, thefirst cycle comprising the second dopant being directed perpendicularrelative to the top plane of the device, and the second cycle comprisingthe second dopant being directed at an non-zero angle relative to adirection of said first cycle; depositing a fourth mask in at least thesecond channel, and increasing a doping concentration of the first layerabutting said portion of the first channel by doping the first channelwith an increased concentration of the first dopants through said windowcreated by the fourth mask; depositing a fifth mask in at least thefirst channel, and increasing a doping concentration of the secondregion abutting said portion of the second channel by doping the secondchannel with an increased concentration of the second dopants throughsaid window created by the fifth mask; depositing one or more additionalinsulating layers, and then re-etching the first channel and the secondchannel; depositing a first contact layer and a second contact layerinto the first and the second channels, respectively; depositingelectrical contacts atop said first and second contact layers; forming awaveguide rib, said waveguide rib projecting from the light carryinglayer, said waveguide rib guiding optical signals propagating throughthe device; wherein the light carrying layer, the first and the secondcontact layers, and the first and the second layers form a lightcarrying stratum; and wherein the p-n junction comprises at least threeportions of an n-type layer, the first portion of the n-type layer beingon a top surface of a p-type layer, a second portion of the n-type layerbeing connected to the first portion of the n-type layer, the secondportion of the n-type layer being disposed on a first side of the p-typelayer, the second portion of the n-type layer contacting a first side ofthe p-type layer, the second portion of the n-type layer extending belowthe top surface of the p-type layer, and the third portion of the n-typelayer being disposed on a second side, opposite the first side, of thep-type layer, the third portion of the n-type layer extending below thetop surface of the p-type layer, the third portion of the n-type layercontacting the second side of the p-type layer, the third portion of then-type layer further extending into the waveguide rib and contacting thefirst insulating layer.
 2. The method of claim 1, wherein the p-typeregion is converted to an n-type region such that the n-type regionextends continuously from the bottom portion of the second channel to aside of the rib.
 3. The method as claimed in claim 1, wherein the firstand the second dopants are activated by a rapid thermal annealing at1000° C.
 4. The method as claimed in claim 1, wherein the p-n junctionremains in the same position with respect to the rib due to a selfaligned fabrication process, the self aligned fabrication process beingmodifiable to select a desired position of the p-n junction with respectto the rib.
 5. The method as claimed in claim 1, wherein some or all ofsaid doping steps are performed at a non-zero, non-perpendicular anglesuch that the dopants are introduced into the rib at a predeterminedposition, self aligned to a position of an edge of a waveguide.
 6. Themethod as claimed in claim 1, further comprising increasing yield byproviding a self-aligned rib fabrication process.
 7. The method asclaimed in claim 1, wherein the first dopant is an n-type dopantselected from phosphorus, antimony, or arsenic.
 8. The method as claimedin claim 1, wherein the second dopant is p-type dopant boron.
 9. Themethod as claimed in claim 1, further comprising providing electro-opticbandwidth of the electro-optic device above 50 GHz.
 10. The method asclaimed in claim 1, wherein the first dopant is p-type dopant boron. 11.The method as claimed in claim 1, wherein the second dopant is an n-typedopant selected from phosphorus, antimony, or arsenic.